Bug 22789: (follow-up) Fix atomic update, GUI and more than one hold

Bug 22789: (follow-up) Fix atomic update, GUI and more than one hold

This patch
* sets one check for reserves and another for old_reserves in
atomic update
* Adds a message below the checkbox and adds detail when a hold is non
  priority
* Fixes issue when there are more than one hold, but the first is non
  priority
* Adds test case for this last scenario

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>